DESCRIPTION: Breast parenchymal patterns are depicted on mammograms as variations in radiographic density, which correspond to the relative amounts of fatty tissue (c.f., epithelial and stromal tissues). Mammographic density is highest in women with the greatest proportion of epithelial, stromal and connective tissues. Compared to no density, high density (>50%) has been consistently associated with significantly elevated long-term breast cancer risk, independent of age, menopausal status, or other breast cancer risk factors. Recently, several small case series have suggested that postmenopausal HRT may increase density in some postmenopausal women, although selection biases and imprecise measurement of exposure and outcomes (density) detract from the validity of these results. Given the small but persistent association of HRT with increased risk of breast cancer, and the increasing prevalence of HRT use among postmenopausal women, assessing the magnitude and correlates of the effect of HRT on mammographic density may contribute to improved understanding of the aetiologic role of exogenous hormones and to public health breast cancer prevention efforts. The objectives are to: 1) reliably estimate the quantitative effect of HRT on mammographic density in postmenopausal women; and 2) determine whether HRT-related density changes differ by ethnicity, age, time since menopause, body mass, or other breast cancer risk factors. This research is ancillary to the WHI, a long-term, multi-center, randomized trial of HRT in postmenopausal women. WHI participants are assigned to HRT (estrogen only for hysterectomized women, or combined progestin-estrogen for women with a uterus) or placebo. Working with the WHI clinical centers, measurements will be made of the percentage of breast density on participants' mammograms taken at baseline, one-year and two-year follow-up intervals, and then compared for longitudinal density change among treatment groups. The sample is comprised of 1200 women with adequate numbers in four ethnic groups: European, African, Hispanic and Asian/Pacific Islander Americans.